In general, acicular metal particles are used in the production of magnetic recording media, and granular or spherical metal particles are also used for specific utilities. These acicular metal particles are very fine particles in which the particle diameter (in the short axis direction) is in the range of from 150 to 500 .ANG.preferably in the range of from 200 to 300 .ANG., and the axis ratio (short axis:long axis) is generally in the range of from 2 to 15. The specific surface area (S.sub.BET) of such acicular metal particles is at least 25 m.sup.2 /g and preferably at least 30 m.sup.2 /g.
Various methods for the production of such magnetic metal powders are known, and can be exemplified as follows:
(1) A method comprising heat-decomposing an organic acid salt of ferromagnetic metal, followed by reducing with a reducing gas, as disclosed in, for example, Japanese Patent Publication Nos. 11412/61, 22230/61, 14809/63, 3807/64, 8026/65, 8027/65, 15167/65, 16899/65 (corresponding to U.S. Pat. Nos. 3,186,829), 12096/66, 14818/66 (corresponding to U.S. Pat. Nos. 3,190,748), 24052/67, 3221/68, 22394/68, 29268/70, 4471/69, 27944/69, 38755/71, 4286/72, 38417/72, 41158/72, and 29280/73, and Japanese Patent Application (OPI) No. 38523/72 (the term "OPI" as used herein means a "published unexamined Japanese patent application");
(2) A method comprising reducing acicular oxyhydroxide alone or in combination with other metals, or reducing acicular iron oxides obtained from such oxyhydroxide, as disclosed in, for example, Japanese Patent Publication Nos. 3862/60, 11520/62, 20335/64, 20939/64, 24833/71, 29706/72, 30477/72 (corresponding to U.S. Pat. Nos. 3,598,568), 39477/72, 24952/73, and 7313/74, and Japanese Patent Application (OPI) Nos. 5057/71 (corresponding to U.S. Pat. Nos. 3,634,063), 7153/71, 38523/72, 79153/73, 82395/73, and 97738/74, and U.S. Pat. Nos. 3,607,219, 3,607,220, and 3,607,220;
(3) A method comprising vaporizing ferromagnetic metals in a low pressure inert gas, as disclosed in, for example, Japanese Patent Publication Nos. 25620/71, 4131/72, and 27718/72, Japanese Patent Application (OPI) Nos. 25662/73, 25663/72, 25664/72, 25665/72, 31166/73, 55400/73, and 81092/73, and Japanese Patent Publication Nos. 15320/74 and 18160/74;
(4) A method comprising heat-decomposing a metal carbonyl compound, as disclosed in, for example, Japanese Patent Publication Nos. 1004/64, 3415/65, 16968/70 and 26799/74, and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882;
(5) A method comprising electrically depositing a ferromagnetic metal powder using a mercury cathode, as disclosed in, for example, Japanese Patent Publication Nos. 12910/60, 3860/61, 5513/61, 787/74, 15525/74, 8123/65, 9605/65 (corresponding to U.S. Pat. No. 3,198,717) and 19661/70 (corresponding to U.S. Pat. No. 3,156,650), and U.S. Pat. No. 3,262,812; and
(6) A method comprising adding a reducing agent to a solution of ferromagnetic metal salt to reduce the salt, as disclosed in, for example, Japanese Patent Publication Nos. 20520/63, 26555/63, 20116/68, 9869/70, 14954/70, 7820/72, 16052/72, 41718/72 and 41719/72 (corresponding to U.S. Pat. No. 3,607,218), Japanese Patent Application (OPI) Nos. 1355/72 (corresponding to U.S. Pat. Nos. 3,756,866), 1363/72, 42252/72, 42253/72, 44194/73, 79754/73 and 82396/73, U.S. Pat. Nos. 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556, 3,663,318, 3,669,643, 3,672,867 and 3,726,664, and Japanese Patent Application (OPI) Nos. 91498/73, 92720/73, 106901/74 and 134467/74.
Magnetic metal powders produced by the above-described methods should generally not be exposed to air due to their high spontaneous ignition properties. Thus, various methods have been proposed for the stabilization of magnetic metal powders in air. For example, Japanese Patent Publication No. 5608/76 and U.S. Pat. No. 3,748,119 disclose a method for stabilizing magnetic metal powders by improving their compositions. In addition, a method for chemically stabilizing magnetic metal powders by providing thereon a protective layer is known, as disclosed in Japanese Patent Publication Nos. 54998/77, 4803/80, 13761/81 and 28366/81, and Japanese Patent Application (OPI) Nos. 5057/71, 135835/74, 97739/74, 5797/78, 5798/78, 51703/78, 76958/78, 11703/79, 46509/79, 139508/79, 84036/80, 84037/80 and 84038/80.
However, an important problem arises in preparing magnetic coating compositions using the magnetic metal powders produced by the above-described methods. More specifically, heat is generated and accumulated in magnetic metal powder by friction, impact and so on during production. This increases the temperature of the magnetic metal powder and an oxidation reaction occurs which deteriorates the inherent characteristics of the magnetic metal powder.
Although the above-described phenomenon is not significant in handling a small amount of magnetic metal powder, it becomes a serious problem when a large amount of magnetic metal powder is handled in air under dry condition in the production on a commercial scale. For this reason, magnetic metal powder is usually treated while it is dipped in a solvent. Moreover, attempts have been made to avoid strong friction and impact of magnetic metal powder in the production of magnetic coating compositions.